The present invention relates to a semiconductor device and a battery pack and, more particularly, to a technique effective in controlling, in a semiconductor device for monitoring the condition of a secondary battery, charging of the secondary battery.
With portable devices, typically smartphones, rapidly spreading in recent years, demand for secondary batteries for driving such portable devices has been rapidly increasing. Since secondary batteries are required to drive portable devices for a long period of time, large-capacity secondary batteries have been developed by many enterprises and organizations. In using a secondary battery to drive a portable device for a long period of time, controlling not only the capacity but also the battery level of the secondary battery is important. To control the battery level of a secondary battery, a method dependent on detection of the output voltage of the secondary battery has been in use. Since, however, it is a characteristic of secondary batteries that their output voltages are approximately constant over their normal operating ranges, the battery level of a secondary battery cannot be accurately controlled by the method in which the output voltage of the secondary battery is detected. Hence, it has been difficult to drive a portable device for a long period of time even by using a large-capacity secondary battery. Under such circumstances, it is currently a mainstream approach to install a “battery monitoring system” on the battery pack side to detect the charge/discharge current of a secondary battery and accurately control the battery level of the secondary battery. A battery monitoring system generally includes an FG-IC (fuel gauge IC), a sense resistor, a MOS transistor for charge control, a MOS transistor for discharge control, and a protection circuit (e.g. a thermistor, secondary protection IC, or fuse) and is provided in a battery pack along with a secondary battery. For example, the FG-IC includes a microcontroller and has functions for controlling the battery level of a secondary battery, controlling MOS transistors for charge and discharge control, and providing primary protection for the secondary battery. The sense resistor is used to detect the charge current and discharge current of the secondary battery as voltage information. The sense resistor for current detection is required to be of high accuracy and a low resistance. Generally, therefore, such a sense resistor is not a resistor element which is formed in the FG-IC using a semiconductor process involving significant accuracy variation. An external high-accuracy resistor element is used as such a sense resistor. The protection circuit such as a thermistor, secondary protection IC, or fuse monitors the condition of the battery and, if any abnormal condition is detected, stops charging or discharging of the battery by turning off the MOS transistor for charge or discharge control so as to prevent the battery from entering a dangerous condition. Existing techniques concerning secondary battery protection functions are disclosed, for example, in Japanese Unexamined Patent Publication Nos. 2005-312140 and Hei 09 (1997)-285033 hereinafter referred to as Patent Document 1 and Patent Document 2, respectively).
As is known from the fact that battery monitoring systems are provided with many protection functions, secondary batteries are dangerous. For example, if a secondary battery, typically, a lithium-ion battery is charged by applying an unnecessarily large current or an excessively high voltage, it may rupture resulting in a serious accident. Therefore, charging a secondary battery in an inappropriate manner must be avoided. This is all the more so, because inappropriate charging of a secondary battery can degrade the secondary battery resulting in shortening the battery life. Hence, besides the protection functions provided in the battery monitoring system as described above, a charge control device like the one disclosed in Japanese Unexamined Patent Publication No. 2006-311799 is provided on the charger side of the secondary battery. For example, as such a charge control device, an IC for charge control compatible with constant-current, constant-voltage charging (hereinafter referred to as CCCV charging) is widely used. In a main-stream configuration of ICs for charge control, analog control is performed as in a DC/DC converter. In CCCV charging, a battery is charged, while its battery level is low using a constant current and, when the battery nears a fully charged state, the battery is charged in a manner to keep the battery voltage constant. To perform CCCV charging of a battery, a sense resistor for current detection is required so that whether the charge current is at an appropriate level can be monitored. The sense resistor, like the one provided in the above battery monitoring system, is also required to be of high accuracy and a low resistance. Therefore, a relatively expensive external resistor element is used as such a sense resistor. Also, in many cases, the charge control device is provided with, in addition to the IC for charge control and the sense resistor, an external transistor for charge control which can turn on/off charging.